Multi-layered anti-reflection coating

ABSTRACT

A multi-layered anti-reflection coating is provided having six or seven layers. The design can be defined by the following parameters: 
     
         ______________________________________                                    
 
    
     N 1  &lt; 1.5            N 1  d 1  = 0.25λ 0              
N 2  &gt; 1.8   0.11λ 0  &lt;                                      
                         N 2  d 2  &lt; 0.19λ 0              
N 3  &lt; 1.7   0.02λ 0  &lt;                                      
                         N 3  d 3  &lt; 0.08λ 0              
N 4  &gt; 1.8   0.12λ 0  &lt;                                      
                         N 4  d 4  &lt; 0.23λ 0              
N 5  &gt; 1.5   0.28λ 0  &lt;                                      
                         N 5  d 5  &lt; 0.36λ 0              
N 6  &lt; 1.5   0.04λ 0  &lt;                                      
                         N 6  d 6  &lt; 0.14λ 0              
N 7  &gt; 1.5   0 ≦                                                
                         N 7  d 7  &lt; 0.55λ 0              
N 4  &gt; N 5  &gt; N 6                                                
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     wherein N refers to the refractive index, d refers to the physical thickness with their respective subnumbers referring to the layer, with the first layer being the layer furthest from the substrate, and λ 0  refers to a preselected design wavelength.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a multi-layered anti-reflection coatingto be applied to a substrate, and more particularly to ananti-reflection coating for use with a glass substrate to reduce thereflectance of light.

2. Description of the Prior Art

A considerable number of multi-layered anti-reflection coatings havebeen suggested in the prior art for a primary design purpose of ensuringthat residual reflectance will be held to a relatively small value overthe entire range of the visual spectrum. For example, multi-layeredanti-reflection coatings having six or seven layers are suggested inJapanese Laid-Open patent application Nos. 56-55902, 52-76943, and54-58037, and U.S. Pat. No. 3,960,441.

On the other hand, various types of automatic focus detecting device forcameras have been proposed recently in which a focusing condition of anobjective lens is detected automatically. Here, a so-called active typeautomatic focusing condition detecting device is suitable for correctlydetecting the focusing condition of the objective lens when the camerais directed to a dark object. The active type automatic focusingcondition detecting device is constructed so that illuminating light isprojected from the device side to a target object to be focused and thefocusing condition is detected by receiving the light reflected by thetarget object.

Here, if such an active type automatic focusing condition detectingdevices is mounted on a single lens reflex camera, it is desirable thatthe illuminating light be projected through the objective lens and thatthe light reflected on the target object be received by the lightreceiving means through the objective lens, in order to decrease theparallax between the objective lens and the optical system of the activetype automatic focusing condition detecting device. Furthermore, awavelength of the illuminating light should be selected within theinfrared light range or near infrared light range, in order to avoid theblinking of the target person caused by stimulation by the illuminatinglight of the eyes of the target person. If the blinking of the targetperson can not be avoided, an undesirable photograph in which eyes ofthe target person are closed is taken.

However, the proposed conventional designs of multi-layeredanti-reflection coatings as described above have sufficientanti-reflection effect with respect to the incident light havingwavelengths within visible light range. Therefore, if the above activetype automatic focusing condition detecting device is mounted on acamera when using an objective lens in which such conventionalmulti-layered anti-reflection coating is provided, most of illuminatinglight emitted behind the objective lens is reflected on one of the lenssurfaces in the objective lens without illuminating the target object,or most of the light reflected from the target object is reflected onone of the lens surfaces in the objective lens without being incident onthe light receiving means of the automatic focusing condition detectingdevice. Thus, it is difficult to correctly detect the focusing conditionof the objective lens with respect to a target object positionedrelatively far from the camera.

SUMMARY OF THE INVENTION

An object of the present invention is to provide an anti-reflectioncoating effective not only for visible light but also for the infraredlight or for near infrared light.

The other object of the present invention is to provide ananti-reflection coating, effective for correctly detecting a focusingcondition of an objective lens with respect to a target objectpositioned relatively far from the camera, when the above active typeautomatic focusing condition detecting device is mounted on a camera fordetecting the focusing condition by using light passed through theobjective lens.

In order to achieve the above described objects, an anti-reflectioncoating according to the present invention comprises at least sixlayers, the first layer furthest from the substrate having a refractiveindex N₁ and an optical thickness N₁ d₁, the second layer having arefractive index N₂ and an optical thickness N₂ d₂, the third layerhaving a refractive index N₃ and an optical thickness N₃ d₃, the fourthlayer having a refractive index N₄ and an optical thickness N₄ d₄, thefifth layer having a refractive index N₅ and an optical thickness N₅ d₅,and the sixth layer having a refractive index N₆ and an opticalthickness N₆ d₆, wherein the refractive indices and the opticalthicknesses are related as follows with respect to a preselected designwavelength λ₀ selected within a wavelength range of 400 nanometers (nm)to 700 nanometers (nm): ##EQU1##

The features of the present invention which are believed to be novel areset forth with particularity in the appended claims. The presentinvention, both as to its organization and manner of operation, togetherwith further objects and advantages thereof, may be best understood byreference to the following description, taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a schematic cross sectional view of anti-reflection coatingsaccording to embodiments 1, 3, 4, 5, 6, 7, 8 and 9 of the presentinvention;

FIG. 1B is a schematic cross sectional view of an anti-reflectioncoating according to embodiment 2 of the present invention;

FIG. 2 is a reflectance graph for the embodiments 1, 2, 3, 4 and 5; and

FIG. 3 is a reflectance graph for the embodiments 6, 7, 8 and 9.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following specification, taken in conjunction with the drawings, setforth the preferred embodiments of the present invention in such amanner that any person skilled in the optical field can use theinvention. The embodiments of the invention disclosed herein are thebest modes contemplated by the inventors for carrying out theirinvention in a commercial environment, although it should be understoodthat various modifications can be accomplished within the parameters ofthe present invention.

Referring to FIGS. 1A and 1B, schematic cross-sectional views of thestructure of the anti-reflection coating of the present invention aredisclosed. An anti-reflection coating C is applied to the surface of asubstrate B such as a glass with the outermost layer being adjacent anair medium A. The anti-reflection coating C shown in FIG. 1A comprisesat least six layers, a first layer 1 furthest from the substrate B, asecond layer 2, a third layer 3, a fourth layer 4, a fifth layer 5, anda sixth layer 6, in sequence from the air side to the glass substrateside. Here it is possible to provide a seventh layer 7 as shown in FIG.1B.

The first layer 1 is made of a material having a low refractive indexless than 1.5, such as MgF₂, and is 0.25λ₀ in optical thickness withrespect to a design wavelength λ₀ selected within a wavelength range of400 nanometers (nm) to 700 nanometers (nm). The second layer 2 is madeof a material having a high refractive index over 1.8, such as a mixtureof ZrO₂ and TiO₂, and is within a range of 0.11λ₀ to 0.19λ₀ in opticalthickness. The third layer 3 is made of a material having a lowrefractive index less than 1.7, such as MgF₂, and is within a range of0.02λ₀ to 0.08λ₀ in optical thickness. The fourth layer 4 is made of amaterial having a high refractive index over 1.8, such as a mixture ofZrO₂ and TiO₂, and is within a range of 0.12λ₀ to 0.23λ ₀ in opticalthickness. The fifth layer 5 is made of a material having a refractiveindex over 1.5, such as Al₂ O₃, and is within a range of 0.28λ₀ to0.36λ₀ in optical thickness. The sixth layer 6 is made of a materialhaving a low refractive index less than 1.5, such as MgF₂, and is withina range of 0.04λ₀ to 0.14λ₀ in optical thickness. The seventh layer 7,if provided, is made of a material having a refractive index over 1.5,such as Al₂ O₃, and is less than 0.55λ₀ in optical thickness.

The optical thicknesses of respective layers are set to optimum valuesso that a sufficient anti-reflection effect may be obtained for aparticular glass substrate B to be used in visible and infraredwavelength ranges, by applying a known method, for example, disclosed inU.S. Pat. No. 4,387,960.

Listed below are definite constructions of various embodiments accordingto the present invention, in which the design wavelength λ₀ isrepresented by 550 nanometers (nm).

In embodiments 1 to 9 listed in Tables 1 to 9 respectively, MgF₂ with arefractive index of 1.385 is used for the first, third and sixth layers1, 3, 6, for its stable optical characteristics and its good resistanceto humidity and physical scratches; a mixture of ZrO₂ and TiO₂(refractive index of 2.10) is used for the second and fourth layers 2,4, for its stable optical characteristics and its good resistance to theeffects of humidity, and Al₂ O₃ (refractive index of 1.62) is used forthe fifth and seventh layers 5, 7 (the seventh layer is omitted inembodiment 2), for its good resistance to the effects of humidity andphysical scratches, respectively. The optical thickness of each layer isset to an optimum value determined by the known method.

The anti-reflection coating is produced by evaporating Al₂ O₃ for theseventh layer (excepting embodiment 2), MgF₂ for the sixth layer, Al₂ O₃for the fifth layer, a mixture of ZrO₂ and TiO₂ for the fourth layer,MgF₂ for the third layer, a mixture of ZrO₂ and TiO₂ for the secondlayer, and MgF₂ for the first layer in that order on the surface of thepreselected glass substrate.

Here, in all of the Tables 1 to 9, the optical thickness of the firstlayer is not represented by 0.25λ₀. However, if the design wavelength λ₀is selected for satisfying N₁ d₁ =0.25λ₀, all of the optical thicknessesof the other layers satisfy the above ranges of optical thicknesses,respectively.

FIG. 2 shows the percent of reflectance versus wavelength curves forembodiments 1 to 5 listed in Tables 1 to 5 respectively. Curves 1 to 5in FIG. 2 indicate the spectral reflectance characteristics ofembodiments 1 to 5, respectively. As is clear from FIG. 2, by adjustingonly the optical thicknesses of respective layers whose material ispreselected above, an anti-reflection coating having a broadanti-reflection spectral range of 400 nanometers to 800 nanometers andminimized residual reflectance less than about 1% is obtainable for anyone of a number of glass substrates whose refractive indices widelyvary.

Embodiments 6 to 9 listed in Tables 6 to 9 respectively are designed forobtaining a sufficient anti-reflection effect within a visiblewavelength range of 400 nanometers to 700 nanometers. FIG. 3 shows thepercent of reflectance versus wavelength curves for embodiments 6 to 9listed in Tables 6 to 9 respectively. Curves 6 to 9 in FIG. 3 indicatethe spectral reflectance characteristics of embodiments 6 to 9,respectively. As is clear from FIG. 3, by adjusting only the opticalthicknesses of respective layers whose material is preselected above, ananti-reflection coating having a broad anti-reflection spectral range of400 nanometers to 700 nanometers and minimized residual reflectance lessthan about 0.6% is obtainable for any one of a number of glass substratewhose refractive indices widely vary.

Furthermore, the anti-reflection coatings of the present embodiments arecomposed of optically stable and highly durable materials for therespective layers, whereby the coatings is optically stable, easy toproduce, and durable as a whole.

While the preferred embodiments have been disclosed in an enablingmanner to facilitate the reproduction of the present invention, itshould be realized that various modifications can be easily accomplishedby a person skilled in this field, and, accordingly, the presentinvention should be measured solely from the appending claims.

                  TABLE 1                                                         ______________________________________                                        EMBODIMENT 1                                                                                    Refractive                                                                              Optical                                                  Material   index     thickness                                         ______________________________________                                        first layer                                                                            MgF.sub.2    1.385     0.274 λ.sub.0                          second layer                                                                           Mixture of   2.100     0.153 λ.sub.0                                   ZrO.sub.2 and TiO.sub.2                                              third layer                                                                            MgF.sub.2    1.385     0.060 λ.sub.0                          fourth layer                                                                           Mixture of   2.100     0.172 λ.sub.0                                   ZrO.sub.2 and TiO.sub.2                                              fifth layer                                                                            Al.sub.2 O.sub.3                                                                           1.62      0.350 λ.sub.0                          sixth layer                                                                            MgF.sub.2    1.385     0.086 λ.sub.0                          seventh layer                                                                          Al.sub.2 O.sub.3                                                                           1.62      0.548 λ.sub.0                          substrate                                                                              glass        1.5                                                                                     (λ.sub.0 = 550 nm)                     ______________________________________                                    

                  TABLE 2                                                         ______________________________________                                        EMBODIMENT 2                                                                                    Refractive                                                                              Optical                                                  Material   index     thickness                                         ______________________________________                                        first layer                                                                            MgF.sub.2    1.385     0.280 λ.sub.0                          second layer                                                                           Mixture of   2.100     0.142 λ.sub.0                                   ZrO.sub.2 and TiO.sub.2                                              third layer                                                                            MgF.sub.2    1.385     0.071 λ.sub.0                          fourth layer                                                                           Mixture of   2.100     0.147 λ.sub.0                                   ZrO.sub.2 and TiO.sub.2                                              fifth layer                                                                            Al.sub.2 O.sub.3                                                                           1.62      0.376 λ.sub.0                          sixth layer                                                                            MgF.sub.2    1.385     0.059 λ.sub.0                          substrate                                                                              glass        1.6                                                                                     (λ.sub.0 = 550 nm)                     ______________________________________                                    

                  TABLE 3                                                         ______________________________________                                        EMBODIMENT 3                                                                                    Refractive                                                                              Optical                                                  Material   index     thickness                                         ______________________________________                                        first layer                                                                            MgF.sub.2    1.385     0.280 λ.sub.0                          second layer                                                                           Mixture of   2.100     0.142 λ.sub.0                                   ZrO.sub.2 and TiO.sub.2                                              third layer                                                                            MgF.sub.2    1.385     0.071 λ.sub.0                          fourth layer                                                                           Mixture of   2.100     0.147 λ.sub.0                                   ZrO.sub.2 and TiO.sub.2                                              fifth layer                                                                            Al.sub.2 O.sub.3                                                                           1.62      0.376 λ.sub.0                          sixth layer                                                                            MgF.sub.2    1.385     0.059 λ.sub.0                          seventh layer                                                                          Al.sub.2 O.sub.3                                                                           1.62      0.504 λ.sub.0                          substrate                                                                              glass        1.6                                                                                     (λ.sub.0 = 550 nm)                     ______________________________________                                    

                  TABLE 4                                                         ______________________________________                                        EMBODIMENT 4                                                                                    Refractive                                                                              Optical                                                  Material   index     thickness                                         ______________________________________                                        first layer                                                                            MgF.sub.2    1.385     0.270 λ.sub.0                          second layer                                                                           Mixture of   2.100     0.161 λ.sub.0                                   ZrO.sub.2 and TiO.sub.2                                              third layer                                                                            MgF.sub.2    1.385     0.049 λ.sub.0                          fourth layer                                                                           Mixture of   2.100     0.191 λ.sub.0                                   ZrO.sub.2 and TiO.sub.2                                              fifth layer                                                                            Al.sub.2 O.sub.3                                                                           1.62      0.333 λ.sub.0                          sixth layer                                                                            MgF.sub.2    1.385     0.102 λ.sub.0                          seventh layer                                                                          Al.sub.2 O.sub.3                                                                           1.62      0.289 λ.sub.0                          substrate                                                                              glass        1.7                                                                                     (λ.sub.0 = 550 nm)                     ______________________________________                                    

                  TABLE 5                                                         ______________________________________                                        EMBODIMENT 5                                                                                    Refractive                                                                              Optical                                                  Material   index     thickness                                         ______________________________________                                        first layer                                                                            MgF.sub.2    1.385     0.265 λ.sub.0                          second layer                                                                           Mixture of   2.100     0.167 λ.sub.0                                   ZrO.sub.2 and TiO.sub.2                                              third layer                                                                            MgF.sub.2    1.385     0.040 λ.sub.0                          fourth layer                                                                           Mixture of   2.100     0.211 λ.sub.0                                   ZrO.sub.2 and TiO.sub.2                                              fifth layer                                                                            Al.sub.2 O.sub.3                                                                           1.62      0.321 λ.sub.0                          sixth layer                                                                            MgF.sub.2    1.385     0.119 λ.sub.0                          seventh layer                                                                          Al.sub.2 O.sub.3                                                                           1.62      0.289 λ.sub.0                          substrate                                                                              glass        1.8                                                                                     (λ.sub.0 = 550 nm)                     ______________________________________                                    

                  TABLE 6                                                         ______________________________________                                        EMBODIMENT 6                                                                                    Refractive                                                                              Optical                                                  Material   index     thickness                                         ______________________________________                                        first layer                                                                            MgF.sub.2    1.385     0.246 λ.sub.0                          second layer                                                                           Mixture of   2.100     0.179 λ.sub.0                                   ZrO.sub.2 and TiO.sub.2                                              third layer                                                                            MgF.sub.2    1.385     0.034 λ.sub.0                          fourth layer                                                                           Mixture of   2.100     0.191 λ.sub.0                                   ZrO.sub.2 and TiO.sub.2                                              fifth layer                                                                            Al.sub.2 O.sub.3                                                                           1.62      0.329 λ.sub.0                          sixth layer                                                                            MgF.sub.2    1.385     0.070 λ.sub.0                          seventh layer                                                                          Al.sub.2 O.sub.3                                                                           1.62      0.532 λ.sub.0                          substrate                                                                              glass        1.5                                                                                     (λ.sub.0 = 550 nm)                     ______________________________________                                    

                  TABLE 7                                                         ______________________________________                                        EMBODIMENT 7                                                                                    Refractive                                                                              Optical                                                  Material   index     thickness                                         ______________________________________                                        first layer                                                                            MgF.sub.2    1.385     0.255 λ.sub.0                          second layer                                                                           Mixture of   2.100     0.162 λ.sub.0                                   ZrO.sub.2 and TiO.sub.2                                              third layer                                                                            MgF.sub.2    1.385     0.051 λ.sub.0                          fourth layer                                                                           Mixture of   2.100     0.160 λ.sub.0                                   ZrO.sub.2 and TiO.sub.2                                              fifth layer                                                                            Al.sub.2 O.sub.3                                                                           1.62      0.346 λ.sub.0                          sixth layer                                                                            MgF.sub.2    1.385     0.053 λ.sub.0                          seventh layer                                                                          Al.sub.2 O.sub.3                                                                           1.62      0.504 λ.sub.0                          substrate                                                                              glass        1.6                                                                                     (λ.sub.0 = 550 nm)                     ______________________________________                                    

                  TABLE 8                                                         ______________________________________                                        EMBODIMENT 8                                                                                    Refractive                                                                              Optical                                                  Material   index     thickness                                         ______________________________________                                        first layer                                                                            MgF.sub.2    1.385     0.254 λ.sub.0                          second layer                                                                           Mixture of   2.100     0.168 λ.sub.0                                   ZrO.sub.2 and TiO.sub.2                                              third layer                                                                            MgF.sub.2    1.385     0.041 λ.sub.0                          fourth layer                                                                           Mixture of   2.100     0.187 λ.sub.0                                   ZrO.sub.2 and TiO.sub.2                                              fifth layer                                                                            Al.sub.2 O.sub.3                                                                           1.62      0.328 λ.sub.0                          sixth layer                                                                            MgF.sub.2    1.385     0.093 λ.sub.0                          seventh layer                                                                          Al.sub.2 O.sub.3                                                                           1.62      0.291 λ.sub.0                          substrate                                                                              glass        1.7                                                                                     (λ.sub.0 = 550 nm)                     ______________________________________                                    

                  TABLE 9                                                         ______________________________________                                        EMBODIMENT 9                                                                                    Refractive                                                                              Optical                                                  Material   index     thickness                                         ______________________________________                                        first layer                                                                            MgF.sub.2    1.385     0.255 λ.sub.0                          second layer                                                                           Mixture of   2.100     0.173 λ.sub.0                                   ZrO.sub.2 and TiO.sub.2                                              third layer                                                                            MgF.sub.2    1.385     0.033 λ.sub.0                          fourth layer                                                                           Mixture of   2.100     0.215 λ.sub.0                                   ZrO.sub.2 and TiO.sub.2                                              fifth layer                                                                            Al.sub.2 O.sub.3                                                                           1.62      0.313 λ.sub.0                          sixth layer                                                                            MgF.sub.2    1.385     0.120 λ.sub.0                          seventh layer                                                                          Al.sub.2 O.sub.3                                                                           1.62      0.291 λ.sub.0                          substrate                                                                              glass        1.8                                                                                     (λ.sub.0 = 550 nm)                     ______________________________________                                    

What is claimed is:
 1. A multi-layered anti-reflection coating for usewith a glass substrate to reduce reflectance of light comprising atleast six layers, the first layer furthest from the substrate composedof MgF₂, the second layer composed of a mixture of ZrO₂ and TiO₂, thethird layer composed of MgF₂, the fourth layer composed of a mixture ofZrO₂ and TiO₂, the fifth layer composed of Al₂ O₃, and the sixth layercomposed of MgF₂.
 2. A multi-layered anti-reflection coating as claimedin claim 1, wherein the refractive indices and the optical thicknessesof the first to sixth layers are related as follows with respect to adesign wavelength λ₀ selected within a range of 400 nanometers (nm) to700 nanometers (nm): ##EQU2## wherein Ni refers to the index ofrefraction, Nidi refers to the optical thickness and the subnumber irefers to the i-th layer.
 3. A multi-layered anti-reflection coating asclaimed in claim 2, further comprising the seventh layer, composed ofAl₂ O₃, having a refractive index N₇ and an optical thickness N₇ d₇related as follows: ##EQU3##
 4. A multi-layered anti-reflection coatingfor use with a glass substrate to reduce reflectance of light comprisingat least six layers, the first layer furthest from the substrate havinga refractive index N₁ and an optical thickness N₁ d₁, the second layerhaving a refractive index N₂ and an optical thickness N₂ d₂, the thirdlayer having a refractive index N₃ and an optical thickness N₃ d₃, thefourth layer having a refractive index N₄ and an optical thickness N₄d₄, the fifth layer having a refractive index N₅ and an opticalthickness N₅ d₅, and the sixth layer having a refractive index N₆ and anoptical thickness N₆ d₆, wherein the refractive indices and the opticalthicknesses are related as follows with respect to a preselected designwavelength λ₀ selected within a wavelength range of 400 nanometers (nm)to 700 nanometers (nm): ##EQU4##
 5. A multi-layered anti-reflectioncoating as claimed in claim 4, further comprising a seventh layer,having a refractive index N₇ and an optical thickness N₇ d₇, providedbetween the sixth layer and the substrate and wherein the refractiveindex and the optical thickness of the seventh layer are related asfollows: ##EQU5##
 6. A multi-layered anti-reflection coating as claimedin claim 5, wherein the first layer, the third layer and the sixth layerare made of MgF₂.
 7. A multi-layered anti-reflection coating as claimedin claim 6, wherein the fifth layer and the seventh later are made ofAl₂ O₃.
 8. A multi-layered anti-reflection coating as claimed in claim7, wherein the second layer and the fourth layer are made of a mixtureof ZrO₂ and TiO₂.
 9. A multi-layered anti-reflection coating for usewith a glass substrate to reduce reflectance of light comprising sevenlayers, the first layer furthest from the substrate having a refractiveindex N₁ and an optical thickness N₁ d₁, the second layer having arefractive index N₂ and an optical thickness N₂ d₂, the third layerhaving a refractive index N₃ and an optical thickness N₃ d₃, the fourthlayer having a refractive index N₄ and an optical thickness N₄ d₄, thefifth layer having a refractive index N₅ and an optical thickness N₅ d₅,the sixth layer having a refractive index N₆ and an optical thickness N₆d₆, and the seventh layer having a refractive index N₇ and an opticalthickness N₇ d₇, wherein the refractive indices and the opticalthicknesses are related as follows with respect to a preselected designwavelength λ₀ selected within a wavelength range of 400 nanometers (nm)to 700 nanometers (nm): ##EQU6##